Examples of a conventional media-agitation type pulverizer for use in wet pulverization include an agitator mill described in JP 2-10699B (Patent Document 1). The agitator mill described in the Patent Document 1 comprises: a milling body which includes a grinding chamber to be filled at least partly with grinding media and material to be ground and has an inlet for material to be ground and an outlet for crushed material, an agitator shaft having an inner shaft end inside the grinding chamber, and a separating means permitting finished pulverized material to flow out of the grinding chamber to the outlet yet retaining grinding media, wherein the agitator shaft has an end portion formed with a cavity therein which is open at the inner shaft end, the end portion of the agitator shaft includes recesses, distributed around the cavity and spaced from the inner shaft end, through which said grinding media from said grinding chamber may axially flow into the cavity and through the inner shaft end to flow back into the grinding chamber, and the separating means is arranged at least substantially inside the cavity.
In the agitator mill disclosed in the Patent Document 1, irrespective of what kind of special structure the separating device has, the separating device is disposed in the cavity formed in the end portion of the agitator shaft and opened at one end thereof so as to be protected. This provides an advantage that there is almost no opportunity for the grinding media to be brought into collision with the separating device due to the rotation of the agitator shaft. In addition, rotation of the agitator shaft can prevent an action of the separating device from being blocked, i.e., a risk of clogging of the separating device.
However, in this agitator mill, when a circulation flow rate of material slurry is increased, or when material slurry has high viscosity and high concentration, the grinding media are undesirably segregated in a region surrounding distal ends of agitating members (which are not agitator shaft) as an end region of a flow within the grinding chamber, and a region around a screen serving as the separating device, to cause a rise in internal pressure of the grinding chamber, resulting in preclusion of stable operation. Moreover, the segregation of the grinding media causes deterioration in flowability around an outer peripheral portion of the agitator shaft acting as the agitating members. This leads to a problem that a solid content of the material slurry fixedly adheres to recesses in the portion of the agitator shaft acting as the agitating members, or outer peripheries of the distal ends of the agitating members undergo uneven wear due to the segregated grinding media.
Therefore, in order to solve this problem, a pulverizer capable of coping with a large-flow-rate operation, as disclosed in JP 3663010B (Patent Document 2), has been developed. The pulverizer disclosed in the Patent Document 2 comprises: a grinding chamber formed in a hollow cylindrical shape having closed opposite ends; a hollow cylindrical separator provided within the grinding chamber in coaxial relation thereto to radially partition an internal space of the grinding chamber into two chambers consisting of an inner chamber and an outer chamber, wherein the separator has a plurality of slits each formed over the entire circumference thereof to provide fluid communication between the two chambers; an agitator unit provided inside the inner chamber rotatably in a coaxial relation to the grinding chamber, a material supply port providing fluid communication between an inside and an outside of the inner chamber and a material discharge port providing fluid communication between an inside and an outside of the outer chamber, wherein the agitating member is formed in a hollow cylindrical shape, and wherein the agitating member has a recess and a protrusion which are alternately provided on an outer peripheral surface thereof, and an opening provided in a hollow cylindrical portion of the agitating member to penetrate therethrough in a direction from an inside to an outside thereof, and wherein the pulverizer is configured to cause a material and grinding media to mutually flow between the inside and the outside of the agitating member via the opening. The pulverizer is also configured such that a ratio of an axial length (L) to a diameter (D) of the grinding chamber (ratio L/D) is 1.0 or less.
In the pulverizer disclosed in the Patent Document 2, the ratio L/D is set to 1.0 or less, so that it becomes possible to resolve segregation of the grinding media around distal ends of the agitating member. Further, a wall of the hollow cylindrical agitating member is provided with the opening penetrating therethrough in the direction from the inside to the outside thereof, and the screen configured to separate the material slurry from the grinding media is provided around the agitating member, so that it becomes possible to provide a structure capable of preventing a flow from the inside of the agitating member toward the screen from being hindered, i.e., to increase a flow rate without any pressure rise, thereby achieving stable operation.
However, in the pulverizer disclosed in the Patent Document 2, the flow from the inside of the agitating member toward the screen is directionally the same as an exit flow of the slurry separated from the grinding media. Thus, the grinding media are inevitably pressed against the screen, thereby causing a problem that a surface of the screen is significant worn away.
Therefore, JP 5046557B (Patent Document 3) proposes a media-agitation type wet dispenser configured to allow grinding media to easily flow inside an agitating member so as to solve the above problem. The media-agitation type wet disperser disclosed in the Patent Document 3 comprises: a cylindrical chamber; a hollow cylindrical separator which radially partitions an internal space of the chamber into an inner chamber and an outer chamber, and has a plurality of slits each providing fluid communication between the two chambers; a rotary shaft rotatably provided while penetrating through one end of the chamber, a rotor fixed to the rotary shaft and configured to be rotated; a material supply port provided at the other end of the chamber to provide fluid communication between an inside and an outside of the inner chamber; and a material discharge port provided at an outer periphery of the chamber to provide fluid communication between an inside and an outside of the outer chamber, wherein the rotor is composed of a plurality of small rotors each comprising a cylindrical agitating section and a disk-shaped holding section, and wherein the agitating section has a plurality of through-holes each providing fluid communication between an inside and an outside thereof. This structure provides improved flowability of grinding media. However, a flow from an inside of the agitating member toward the screen is directionally the same as an exit flow of the slurry separated from the grinding media. Thus, when a recirculation flow rate is increased, a force acting to return the grinding media toward the inside of the agitating member becomes failing to withstand the exit flow of the slurry, resulting in significant wear of the screen.